Negative feedback circuit arrangement



July 21, 1942.

J. \HAANTJES NEGATIVE FEED-BACK CIRCUIT ARRANGEMENT Filed Jax a. 11, 1941 I; L .9 2 IQ 7' AMPL/HER osc/zumzz 2 MODULATOR" f 1 a AMPLIFIER AMPLIFIER 6 L4 k7 vvvvv vv 40W FREQUENCY nvpur 121 3 Y ;4MPu -'/z-'R MIXER 1.1-. DETECTOR zo w mini/mar 18 AMPLIFIER) v l I I L i 22 43 OSCILLATOR INVENTOR JOHA/V HAANT/ES ATTORNEY lation frequencies. 1

Patented July 21, 1942 UNITED STATES PATENT' O l-FICE' NEGATIVE FEEDBACK omcUrr ARRANGEMENT Johan Haantjes; Eindhoven, Netherlands Application January 11, 1941, Serial No. 374,066 In the Netherlands September 26, i939 4 Claims.

v This invention relates to a circuit arrangement for transmitting amplitude-modulated oscillations through at least one amplifier.

'Such a circuit arrangement is used, for in-- stance, in radio receivers, in radio transmitters and in systems for carrier wave telephony.

The invention has for its object to utilize in such circuits negative feed-back for the modu- It has already been suggested to establish negative feed-back for the modulation frequencies by detecting the output voltage of the amplifier for amplitude-modulated oscillations and by modulating again the modulation-frequencies latter being tuned to the frequency of the am- This oscillatory plitude-modulated oscillations.

circuit is disposed in the input circuit of the amplifier in such manner that the applied bias becomes more positive with an increasing carrier wave amplitude.

The diode acting as a modulator involves amplitude modulation in the oscillatory input circuit, which modulation is additiveiy composed of the amplitude modulation already available and the detected voltage carried back.

In this way the drawbacks referred to above do not occur in this circuit. I

The invention 'will be more clearly understood by reference to the accompanying drawing given by way of example.

Figure 'l-shows a circuit according to the invention:

Fig. 2 shows an application of the invention to a radio transmitter: and

'ceivel- In 1 the referencenumber l denotes an- Fig. 3 shows howit is in a radio reoscillatory circuit 4. A diode 5 having a load resistance is connected to the oscillatory circuit 4. This load resistance consists of two seriesconnected resistances 6 and 1. In parallel withthe resistances Band I is connected a condenser 8 which has 'a' negligibly small impedance to wards the amplitude-modulated oscillations. A

diode 9 having a load resistance is connected in parallel with the oscillatory circuit i inthe input circuit of the amplifier 2. This load resistance consists of the resistance I and a resistance ill connectedin series. In parallel with the re.- sistances 1 and I0 is connected a condenser H having a low impedance for the .oscillations to be amplified. Jhcoperation of this circuit is as follows:

The amplitude-modulated oscillations in the,

oscillatory circuit 4 are rectified by the diode 5. As a result thereof a direct voltage having superposed thereon a low frequency alternating voltage is set up across resistances 6 and l. The

part of the sum of these voltages set up across resistance 1 constitutes a positive bias for the diode 9 which bias varies with the amp tude of the modulation frequency. This results in that the diode 9 exerts a variable damping on. the oscillatory circuit i which damping depends on the instantaneous values of the modulation-envelope. 1

In this way these modulation frequencies in the oscillatory circuit l are modulated again on the oscillations already available. when using the diode 9 this modulation is superposed on the initial modulation and the generation of higher harmonics and combination frequencies is avoided.v The-"initially available modulation is counteracted by the modulation caused by the diode 0. Thus negative feed-back for the modulation frequencies ensues as a result of which the distortion caused in the amplifier is reduced. v. v

This negative feed-back when restricted solely to the low frequency alternating voltage through the diode I is en direct voltage p uced upon detection were not supplied at the same time there would ensue a decrease in modulation depth of the amplitude-modulated oscillationswhich would require stronger amplification.

In Fig. 2 the circuit'arrangement referred to,

' above is used in a radio transmitter toreduc'e the distortion to which the enveloping curve of the amplitude-modulated oscillations is liable in 5 t-ter. Corresponding parts of Figs.

1y sufilcient. If, however, the f amplitude-modulated oscillations are produced in a modulator 12 to which are supplied nonmodulated oscillations by an oscillator I3, and modulation frequencies by a low frequency amplifier I l. The amplitude-modulated oscillations are transmitted to the oscillatory circuit l by an oscillatory circuit I5 connected to-the modulator l2. The load connected to the oscillatory circuit 3 is constituted by a transmitting antenna I! which is inductively coupled with the circuit 3 through a coil [6.

In Fig. 3 the circuit arrangement shown in Fig. 1 is used in a radio receiver where the intermediate frequency amplifier, and in addition the lowfrequency amplifier of the receiver, is back-coupled in a degenerative manner.

- The oscillations picked up by means of an antenna l8 are amplified in a high frequency am-- plifier l9 and jointly with locally generated oscillations of an oscillator supplied to a mixing stage 2l.- The intermediate frequency amplitude-modulated oscillations produced herein are amplifiedin an intermediate frequency amplifier 22 and subsequently detected by a second detector 23, whereupon the low frequency oscillations produced herein are-amplified by a low frequency amplifier 24 and supplied through transformer 25 to a loud speaker 26.

The input circuit of the intermediate frequency amplifier 22 similarly to Fig. 1 includesan oscillatory circuit I, a diode 9, a condenser II and a resistance 10.

For the negative feed-back of the envelopingcurve of the amplitude-modulated oscillations and of the low frequency modulation frequencies two voltages'are supplied in series to the diode 9. One of these voltages is a direct voltage which is a measure of the intensity of the intermediate frequency carrier wave and is produced by detection in the second detector 23 and diverted therefrom to ground by way of a potentiometer 28. A part of the voltage across this potentiometer is supplied in series with a low 1. In a discharge tube amplifier and circuit. 'arrangementtherefor; a variable unilateral impedance of the diode type in shunt with aninput circuit for said amplifier, means for rectifying a modulation component of the output energy from said amplifier, and means for superposing fier, a diode rectifier in shunt withthe resonant frequency voltage to the diode B. This low frequency voltage is taken from the secondary winding of transformer 25 and, through a potentiometer 29 and a phase shifting device 30, connected in series with the direct voltage through part of the potentiometer 28. The potentiometer 29 permits adjustment of the correct value of the various components of the low frequency voltage, while the phase shifting device 30 allows adjustment of their phase.

In fact, frequency dependent phase displace-- merit occurs in the intermediate frequency amplifier and in the low frequency amplifier, which displacement must be corrected prior to backcoupling ina degenerative manner.

Negative feed-back of the whole receiver might be achieved by connecting the diode 9 to the input'oscillatory circuit of the high frequency amplifier l9. In this case, however, the drawback occurs that the incoming signal voltage is therectified modulation component upon the energy shunted through said variable unilateral impedance, thereby to produce a negative feedback effect upon said input circuit.

2. In an amplifier according to claim 1, the combination so defined in which the circuit of said variable unilateral impedance includes a diode rectifier and a potentiometer a portion of which is in circuit with said means for rectifying a modulation component of the output energy.

3. A circuit arrangement for translating amplitude modulated oscillations, including an amplifier, a resonant input circuit for said ampliportion of said input circuit, an electronic detector coupled to the output side of said amplifier, a resistive. impedance interconnecting the respective cathodes of said rectifier and said detector, a direct current connection from the oathode of said diode to ground, a resistive feedback circuit from the anode of said detector to an intermediate tap on the resistive impedance first mentioned, and capacitive means for dissipating unwanted alternating potential components in said input and output circuits; said diode rectifier and said feedback circuit being cooperative to produce a variable damping effect on said amplifier which is 'a function of the amplitude of said modulated oscillations.

'4. A circuit arrangement for receiving amplitude modulated oscillations, including at least .one amplifier, a resonant input circuit for said amplifier, a diode rectifier in shunt with the resonant portion of said input circuit, a detector coupled the output side of said amplifier, a utilization device having an input circuit and means coupling the same to said detector, a feedback circuit connected to the coupling means between said detector and said utilization device,

' the energy shunted through said diode rectifier,

thereby to produce a negative feedback effect on the input circuit of said amplifier.

- JOHAN HAANTJESL 

